1. Field of the Invention
This invention relates to electrical logging apparatus and methods for determining the nature and characteristics of the various sub-surface formations penetrated by a borehole in the earth.
2. Background Art
Various resistivity logging techniques have been used to determine electrical properties of sub-surface formations. One commonly used technique is induction-SFL (spherically focused laterolog) logging. The induction component of the induction-SFL logging measures the resistivity (or its inverse, conductivity) of the formation by inducing eddy currents in the formations in response to an AC transmitter signal. The eddy currents induce secondary magnetic fields that in turn induce a voltage in a receiver antenna. Because the magnitudes of the eddy currents depend on formation conductivities, the magnitudes of the received signal thus reflect the formation conductivities. The SFL component of the induction-SFL tool measures the resistivity by injecting a current into the formation and measures the currents or voltage drop across a pair of measuring electrodes.
To obtain true formation resistivity, the log data need to be corrected for various effects that influence the measurements. For example, the resistivity measurements may have unwanted contributions from currents flowing in the sedimentation layers (beds) lying above and below the layer under investigation. This is referred to as shoulder effects, which are particularly problematic if the layer under investigation is less conductive than the adjacent beds.
To correct the shoulder effects, a sonde response function may be used to correlate the voltage measurements with true formation conductivity. This sonde response function is also known as the vertical sensitivity curve of the induction tool. For homogeneous formations, the sonde response function can best be described as a response curve, which has a main lobe of finite width and one or more sidelobes located on each side of the main lobe. These sidelobes are responsible for the shoulder effects.
Several approaches have been proposed to minimize these sidelobes (hence, the shoulder effects). For example, U.S. Pat. No. 2,582,314 issued to Doll and U.S. Pat. No. 3,067,383 issued to Tanguy disclose induction tools having multiple transmitter and receiver coils arranged in specific relationships to “focus” the sonde response function by narrowing the width of the main lobe and attenuating the sidelobes. In an alternative approach, U.S. Pat. No. 2,790,138 issued to Poupon discloses an induction logging tool having two separate induction coil arrangements, which have the same geometrical center so that responses from the two coil arrangements may be used to cancel the contributions from the sidelobes.
In addition to the shoulder effects discussed above, skin effects may also limit the ability of the induction logging equipment to accurately measure the true conductivity of the formations. The skin effect is characterized by the non-linear responses of the sonde response function as a function of the formation conductivity. The skin effect results primarily from interactions between different eddy currents flowing in adjacent loops in the formation. Prior art has shown that the magnitudes of skin effects depend on a complicated function of the coil system operating frequency, the effective length of the coil system, and the conductivity value of the adjacent formation, among other things.
To minimize the shoulder and skin effects and to design a better induction tool, various factors should be taken into account. These factors include depth of investigation (DOI), resolution, borehole effects, frequency of operation, and mutual inductance. DOI concerns how far the tool can “see” into the formation from the borehole wall. It is desirable that an induction tool is capable of a deep DOI such that the measured formation resistivity is unaffected by mud invasion. Typical invasion radii range from 0 to 4 feet, but can range up to 8 feet or more. To have a deep DOI, the transmitter-receiver spacing needs to be large. However, large transmitter-receiver spacing increases the percentage of non-linearity of the responses resulting from the skin effects. A large transmitter-receiver spacing also increases tool length and cost.
In addition to the ability to “see” deep into the formation, an induction tool should also have high resolution such that the apparent resistivity reading for the bed of interest is less affected by adjacent beds. However, to achieve a high resolution, the transmitter-receiver spacing needs to be small; this reduces the DOI of the tool. Therefore, a compromise is necessary. An alternative to a high resolution tool is to use signal processing to enhance the vertical resolution of the tool. For example, a method for enhancing the vertical resolution of an induction logging tool is disclosed in U.S. Pat. Nos. 4,818,946 and 4,837,517, both issued to Barber and assigned to the assignee of the present invention. These patents are incorporated by reference in their entirety.
An ideal tool should also have little borehole effect in holes with various diameters, e.g., ranging from 8 to 16 inches. Alternatively, if the borehole effect is non-negligible, means for borehole correction should be provided and the correction procedure should be simple. For example, the borehole correction may be achieved by including a sensor, e.g., the Rm sensor on an array induction tool sold under the trade name of AIT™ by Schlumberger Technology Corporation (Houston, Tex.), that provides a measure of the borehole effects.
The operational frequency of the tool has an effect on DOIs and signal-to-noise ratios (SNR). High frequency produces low noise (i.e., better SNR). However, high frequency is more susceptible to skin effects (hence, shallower DOI). Typical prior art induction tools operate at a frequency ranging from tens of KHz to a few MHz.
Mutual inductance between the transmitter and the receiver coils can severely impact the measurable signal magnitudes. Therefore, mutual inductance should be kept as low as possible so that it will not obscure the conductivity signals from the formation. Mutual inductance can be eliminated or minimized by including a bucking coil between the transmitter and the receiver coils. The use of bucking coils in induction tools is well known in the art.
Taking these factors into account, conventional induction tools, such as that described in U.S. Pat. No. 3,179,879, have evolved to use focused multi-coil arrays for measuring resistivities at several DOI. A minimal configuration of such tools includes two coil arrays for measuring at two different DOls: a deep array (ILD) and a medium array (ILM). The multi-coil arrays with different DOI can detect and correct for environmental effects such as borehole effects and mud invasions. For example, the ILD array is designed to see beyond the mud filtrate invaded zone.
In addition to the improvement in tool designs over the last several decades, various signal processing methods have been developed to correct for shoulder effect. Examples of these approaches include phasor processing disclosed in U.S. Pat. No. 4,513,376 issued to Barber and U.S. Pat. No. 4,471,436, issued to Schaefer et al. These patents are assigned to the assignee of the present invention, and they are incorporated by reference in their entirety.
In addition, U.S. Pat. Nos. 4,818,946 and 4,513,376 issued to Barber disclose methods of processing the induction log measurements to reduce the unwanted contributions in the log measurements by minimizing the sidelobes in the sonde response function used to translate the formation conductivity values into the processed measurements.
The efforts to improve tool accuracy have resulted in tools that include many components and circuitries. As a result, the logging tools tend to be long. For example, an induction tool described in U.S. Pat. No. 5,157,605 issued to Chandler et al. has a length of approximately forty feet (see FIG. 1A). The increased length requires more rig-up time to assemble and insert the logging tools in the wellbore and increases the need to drill more rathole (excess footage drilled below the lower most zone of interest to enable the logging tool sensors to be positioned deep enough to acquire data over the lower section of the zone of interest). In addition, a long tool has a tendency of getting stuck in wellbores having poor borehole conditions and cannot be placed into wellbores having severe dog legs or horizontal wells having a short kickoff radius.
Therefore, it is desirable to have shorter resistivity logging tools. One approach to shorten the tool length is to use a folded antenna array as disclosed in U. S. Pat. No. 5,905,379 issued to Orban et al. (shown in FIG. 1B). In the folded antenna arrays, the receiver antennas and the bucking coils are all disposed on one side of the transmitter antenna, instead of on both sides of the transmitter antenna (see FIG. 1A). The folded array significantly reduces the length of the tool, i.e., to about sixteen feet including the associated electronics. The antenna section length is about 8 feet.
Induction array tools provide good performance, but they use shallow-reading induction antenna instead of galvanic electrodes to measure the near wellbore resistivity. In “bad hole” conditions (i.e. washed-out or rugose wellbores), the electrode devices (e.g., an SFL) can provide better resistivity measurements than the shallow-reading induction antennas. In addition, at high resistivities, the electrode devices can provide better resistivity measurements than an induction device. For these reasons, SFL or other shallow electrode devices are more robust. Accordingly, electrode devices are preferred under high resistivity and bad hole conditions, which are often found in low-cost wells.
Another interest in having better induction tools including SFL or electrode devices stems from the fact that many old resistivity logs are acquired with induction-SFL type devices that measured ILD, ILM and SFL. With the recent interests in redeveloping old oil fields that have not been developed because of thin pay zones, there is a new demand for apparatus and methods that not only can provide accurate measurements, but also offer the possibility of well-to-well correlation with the “old”induction-SFL measurements.
Therefore, a need exists for better, but simpler, resistivity logging tools and methods for acquiring induction-SFL measurements.